Download videos from various Flash-based video hosting sites, without having to use the Flash player. Handy for saving videos for watching offline, and means you don't have to keep upgrading Flash for sites that insist on a newer version of the player.

Supports Linux, Windows and OS X - see Installation for more details and how to install.
Includes support for the following sites/players (and more!):

YouTube, eHow, Brightcove (used by many sites like Channel 4, Daily Telegraph ...), BBC (news, etc), Metacafe, 5min, Google, fliqz, nicovideo, vimeo, Blip, Break, Collegehumor, Muzu, Sevenload, Megavideo, Wat.tv.
Also includes a 'generic' method which works on many other sites.
Because a picture is worth a thousand words:

OK, I’ll confess upfront I’ve not tried this one yet although I hope to tomorrow. I got this info from Ars.

The basic problem is when you have a movie you’ve ripped but you want to watch it both on your Apple TV and your iPhone. Now ideally you’d want two different sizes. The iPhone doesn’t need the resolution you’d rip for the Apple TV plus you want the size to be as small as possible. Now if you’ve ever bought a show from iTunes you’ll note that Apple gives you both hi-res and low res versions. How do they do this? Well it turns out it’s just a tag on the MPEG-4 file. And there is a shell script to set those tags so you can do it with your own files.

First download and install mp4v2. Yes, you have to compile it. However if you have MacPorts you can also install it that way.